1. Field of the Invention
The present invention relates to positioning servomechanisms and particularly relates to those feedback servomechanisms for positioning a load relatively to any selectable one of a plurality of rest positions in response to an applied demand signal which use a polyphase position transducer for providing a selectable one out of a plurality of distance-cyclic mutually staggered position indicating signals as the feedback signal. The present invention is applicable to servomechanisms for positioning a transducer at selectable radii on a rotatable information storage disc, which application is illustrative of and not restricting to its field of utility.
2. The Prior Art
It is well known to provide a data storage disc for storing retrievable informational data in a plurality of concentric, data storage tracks on a rotating disc accessed by the placing in adjacence thereto of a read/write transducer for the purpose of recording and/or reading data on or from a selectable one of the tracks. The placing of the transducer is generally achieved by issuing position commands to a transducer positioning servomechanism which responds by moving the transducer to interact with the selected track.
The transducer positioning servomechanism is known to employ an optical grating position transducer coupled to the transducer and providing a plurality of equi-periodic phase-staggered cyclic position indicating signals any one of which is selectable as the position feedback signal in the servomechanism. It has generally been the practice that the position of the transducer relative to the disc has been determined by which one of the position indicating signals is provided as the feedback signal. The transducer therefore has been provided with a plurality of rest positions across the radius of the disc, each one of which is intended to correspond to the transducer being interactive with a selected one of the tracks. It has also been the practice, as track densities (measured in terms of the number of tracks per unit radial distance of the disc) increase, to allow for the provision of a demand signal to the servomechanism so that correction can be made to the position of the transducer as achieved by the provision of one or other of the feedback signals. The higher track densities demand a higher precision in placing the transducer over a track so that the transducer can be prevented from reading from or writing on a data storage track adjacent to the intended track. Various schemes exist whereby a corection signal can be worked out and applied to correct for disc and disc drive mechanical errors and expansion or contraction. These schemes usually involve the use of one or more servo tracks on the disc and rely upon an implied accuracy and predictability of the relative spacing between the plural position indicating signals from the transducer not only at any one radius of the disc but also across the entire surface of the disc where-over the transducer is to be located.
The optical grating position transducer generally comprises a shutter and grating assembly. The shutter generally comprises a glass slide mechanically coupled to the data recording and/or reading transducer to move with it. The grating is generally affixed to the body of the disc drive and comprises a glass grating. Both the shutter and the grating have opaque lines thereon at the same regular spacing on both the lines having a width of half their repetition distance and being aligned at ninety degrees to the direction of travel of the shutter. A lightsource shines light through the combination of the shutter and the grating. The opaque lines on the grating are staggered in their spacing to one another in different areas of the grating. Each of the different areas of the grating is provided with its own photodetector. As the shutter moves each photodetector provides a cyclic, repetitive position indicating signal which is different in phase from the outputs of the other photodetectors. The outputs of the photodetectors can be variously combined with one another to give more desirable position indicating output signals.
The glass shutters and gratings are generally made by photographic processes. The exact widths of the opaque lines are dependent upon exposure and development times and therefore posess an element of unpredictability. The masters for the photographic process are generally photographic reductions of hand or machine produced masters themselves subject to errors when prepared and further subject to errors when photo-reduced. The shutters and masks are generally batch-prepared, there being many shutters and/or gratings on each sheet of glass subjected to the photographic process. The divergence of the light from the source used for exposure of the glass during the manufacturing process from parallelism means that different shutters and/or gratings have different angles of shadows along their lengths, making their pitches uneven not only with respect to one another but also with respect to different parts of themselves. In use the lightsource in the position indicating transducer may not be parallel and may cast shadows of the opaque lines in slightly different positions in each of the areas of the grating so that the resultant position indicating signals are not quite in their intended positions relative to one another. The circuits used for processing the outputs of the photodetectors can be subject to gain and offset errors causing mispositioning which varies with each signal used as the feedback signal. During the travel of the read/write transducer across the surface of the disc the alignment and spacing of the shutter relative to the grating can change altering the size and relative spacing of the different position indicating signals. All of the above effects ensure that the spacing between the position-indicating signals cannot be guaranteed at any point in the travel of the read/write transducer across the disc nor can the errors which are detectable at any one point be assured of remaining the same at all others.
It is therefore desirable to provide a positioning servomechanism wherein a selectable one of a plurality of equi-periodic spatially staggered position-indicating signals is usable as the feedback signal and wherein errors between the relative phases of the plurality of position indicating signals can be detected and compensated for across the extent of the positioning range of the servomechanism. It is further desirable that such a servomechanism be applicable to read/write transducer positioning in a disc data store.
The trend towards higher radial track densities requires the provision of glassware whereon the opaque lines are increasingly thinner and closer together. It is inconvenient and expensive to manufacture glassware of such fineness, and difficult and expensive to provide mounting for such glassware to be used in a position-indicating transducer. The angular error between opaque lines can cause large variations in transducer signal amplitudes and serious rounding of the ideally triangular output waveform. It takes much effort and time to establish the optimum manner of manufacture for a particular set of glassware and it is inconvenient to have to repeat this exercise each time it is desired to alter the radial track density. It is therefore desirable to provide a positioning servomechanism wherein the spacing between adjacent rest positions for the load can be rendered independent of the spacing of the cyclic position-indicating signals from a transducer and independent of any phase errors therebetween. It is further desirable that such a positioning servomechanism be employable for positioning a read/write transducer at selectable radii on a rotatable disc in a disc data store.